Many types of semiconductor devices such as dynamic random access memories (DRAMs), static rams (SRAMs), and programmable read-only memories (PROMs) are formed in much the same way. Layers of oxide, nitride, and polycrystalline silicon (polysilicon or poly) are formed over a substrate such as monocrystalline silicon or gallium arsenide to form field and gate oxide, capacitor cell plates, word and digit lines, and various other structures.
To form a thick oxide layer 10 such as field oxide shown in FIG. 1, a thin blanket oxide layer 12 is formed over a substrate 14. A blanket layer of protective material such as nitride 16A-16D is formed over the oxide layer 12, and a patterned mask (not shown) is formed over the protective layer to cover regions of protective material and leaving other regions of protective material exposed. The exposed protective regions (and often the underlying oxide layer) is etched to expose the underlying oxide layer (or underlying substrate). The substrate is then oxidized to form the field oxide as shown in FIG. 1.
Depending on the design of the device being formed, the field oxide can often require varying distances between an adjacent field oxide. To form the field oxide, the protective structures formed have varying distances between them as shown in FIG. 1. The distance between protective structure 16A and protective structure 16B, for example, is smaller than the distance between protective structure 16B and protective structure 16C. One problem which can result from differing distances between protective structures is field oxide thinning. When the substrate is oxidized, the smaller distance between protective structures results in a thinner field oxide than in regions that have a larger distance between protective structures, such as shown in FIG. 1. This can result in a device that has less desirable electrical characteristics than a device in which the field oxide thickness is uniform across the substrate. A process for reducing field oxide thinning would be desirable.